cookies exercise hint/apis list improvements

[jasonpowers]

By introducing ctx with no explanation, the developer is required to go out and read the koa documentation. This wouldn't be an issue, but the expressjs/cookies page linked in the exercise doesn't have any documentation on how to use cookies with koa, only express, and the link off of the koa documentation page is different (but ends up in the same place). Initially I was under the false impression that I needed to require() it in. Perhaps linking in the koa cookie docs under the hint would help clarify.